1. Field of the Invention
This invention relates to U. V. stabilized articles, and, more particularly, to a PVC resinous sheet having its surface modified with a U. V. stabilizer.
2. Description of the Prior Art
Photodegradation of many types of solid polymeric articles by U. V. light has been a continual problem confronting the industry. Such U. V. degradation causes discoloration, embrittlement and deterioration of the article. Accordingly, U. V. absorber compounds have been incorporated into such polymeric articles in an attempt to provide effective stabilization to U. V. light.
However, since photodegradation is known to be a surface phenomenon, such incorporation of U. V. stabilizers into the polymeric material prior to shaping into solid form has been largely uneconomical and inefficient because the stabilizer is present substantially in the interior of the shaped article, where it does not function effectively to prevent surface photodegradation.
Accordingly, surface treatments have been proposed to provide more effective U. V. protection for such resins. These treatments are based on the ability of various organic solvents to swell such resins as polyvinyl chloride. This swelling property enables the ultraviolet light stabilizer material to be directed principally into the surface region of the resin body. However, none of these surface treatments has been very successful commercially. Another surface method comprises laminating the surface of the polymer with a layer of a film-forming material containing the U. V. stabilizer. This lamination approach, unfortunately, suffers from the problems of incompatibilities between polymer, U. V. stabilizer, and the film-forming material and its carrier components. Furthermore, the laminated layer and the surface of the polymer do not adhere well, and delamination upon shock or impact is a serious problem.
The swelling method is described by Katz, et al. in an article in the "Proceedings of the A. C. S. Div. of Org. Coatings and Plastics" 36, (1) p. 202-205 (1976), entitled "Ultraviolet Protection of Transparent PVC by Diffusion Coatings". Katz uses a dip method to impregnate a transparent PVC sheet to a given depth with a non-migrating ultraviolet light absorber. The dip technique comprises swelling the polymer with a carrier solvent, infusing the stabilizer from a solution thereof with the carrier, and, thereafter heating the treated polymer in air at an elevated temperature to drive the stabilizer below the surface of the polymer and to remove residual solvent. Under these process conditions, however, only a small amount of stabilizer is absorbed into the surface region. Furthermore, the dip method produces a product in which both sides of the sheet are modified with ultraviolet absorber.
Jochanan, in Israeli Patent No. 39037, published Nov. 10, 1975, entitled "U. V.- and Oxidation-Proof Products from Organic Plastics and their Manufacture", describes various methods of applying a solution or suspension of an ultraviolet light absorber to the surface of a plastic sheet. The methods disclosed by Jochanan include immersion, spraying, brushing, roller-printing and curtain-coating. However, with such methods, a large amount of residual stabilizer and solvent is left on the surface of the article, whereupon the modified surface is observed to be streaky, hazy, and has runs or pockmarks thereon.
Humphrey, in U.S. Pat. No. 4,146,658, describes a process for producing an ultraviolet light stabilized polycarbonate article by dipping the polycarbonate sheet into an impregnating solution consisting of a selected combination of ultraviolet absorber and solvent. However, all the disadvantages of the conventional dip method are present here, too.
Ching, in U.S. Pat. No. 4,349,607, describes the incorporation of certain U. V. stabilizers into thermoplastic resins by dipping spraying and brushing followed by heating at 70.degree. C., which method is unsatisfactory for providing surface modified PVC sheets having a high degree of surface perfection.
Amborski, in U.S. Pat. No. 3,043,709, discloses an organic polymeric article having an ultraviolet light absorber compound incorporated adjacent to the surface fo the article. The article is treated by several methods, including dipping-coating a polyethylene terephthlate film in a stabilizer solution or, applying a dispersion or melt of the absorber, or applying the absorber as a powder, and, thereafter, the coated film is heated in air to 115.degree. C. and above to drive the absorber below the surface of the film and to remove the solvent. These processes, however, are disadvantageous because they require heat-treatment to fully modify the surface of the film. Furthermore, these methods form an article in which considerable residual absorber is present on both sides of the film, which affects its surface perfection and appearance.
Fanning, in U.S. Pat. No. 3,297,462, discloses a process for rendering polymeric films resistant to the deteriorating effects of sunlight by immersing them in a solution of a benzophenone stabilizer in a solvent mixture of diethylene glycol and glycerine, and then washing successively in diethylene glycol and water. The polymers suitable for such treatment, however, must have a zero strength temperature of at least 200.degree. C., which excludes many useful polymers, including polyvinyl chloride.
Solvay and Cie, in Belgium Patent No. 612,206, discloses a process for surface treating rigid polyvinyl chloride objects. The method comprises immersing the object in a swelling solvent containing a light stabilizer and then evaporizing the solvent in air. This procedure, however, is ineffective because some stabilizer remains on the treated surface of the object, which gives a poor physical appearance, and, environmental hazards are created during removal of the solvent in air.
Cohnen, in U.S. Pat. No. 3,892,889, describes a method of applying a solution of a U.V. stabilizer to polycarbonate resin sheets. Such treated polymers, however, also are marred by the presence of residual stabilizer on the surface of the article.
Chilton, in U.S. Pat. No. 4,323,597 teaches a flow-coating process for polycarbonate articles. In this process, the surface of the polycarbonate is heated and a liquid stabilizer composition, which is non-aggressive towards but wets the polycarbonate surface, is flowed over the heated surface. There is no indication in this patent, however, that the method is applicable to polyvinyl chloride.
Bristol, in U.S. Pat. No. 3,519,462, describes the treatment of polyvinyl chloride and other polymers with a solution of a U.V. stabilizer in a diol or triol. These solvents, however, are not efficient swelling agents for PVC. Furthermore, there is no recognition therein of the need for removing excess stabilizer solution present on the treated surface.
Chauffoureaux, in U.S. Pat. No. 3,783,011, diffuses a vapor of a U.V. stabilizer into the surface of polyvinyl chloride and other polymers. Unfortunately, the vapor treated surfaces have poor physical characteristics.
In summary, these and other processes have not provided useful U.V. stabilized articles, particularly polyvinyl chloride resin articles, for example, for residential siding, which use requires particularly effective protection from the weathering effects of sunlight, and an appearance and degree of surface perfection which is acceptable in commercial use.
Accordingly, there is a present need to effectively modify the exposed surface of a PVC sheet, such as residential siding, with a U.V. stabilizer. Such a surface modified, U.V. stabilized PVC siding article preferably should have only one side surface modified with the stabilizer material. Furthermore, the exposed surface of the sheet should be substantially free of residual stabilizer and solvent material, and the appearance and degree of surface perfection of the modified side should be the same as that of the unmodified side of the sheet. In addition, the surface modification should be effected in situ to lesson environmental hazards.
Accordingly, it is an object of this invention to provide a surface modified, U.V. stabilized resin sheet article having both sides exposed to the air, wherein the surface is substantially fully modified in situ with a stabilizing amount of a U.V. stabilizer material, the exposed surfaces are substantially free of residual stabilizer and solvent material, and the appearance and degree of surface perfection of the modified side are substantially the same as that of the unmodified side.